The present invention is in the area of internal combustion engines and the combinations of such engines with other devices for increasing the efficiency of converting energy in fuels to useful work. It is more particularly in the area of piston-type, compression-firing engines and peripheral devices. The invention has broad application, and is particularly suited to aircraft propulsion applications.
Internal combusion piston engines were first developed in the nineteenth century, and were successfully applied to stationary applications and land vehicle propulsion before the end of that century. Early in the twentieth century such engines were first applied to powering heavier-than-air aircraft. The successful flight by the Wright brothers at Kitty Hawk, N.C., in 1907 is an example. Piston engines of many sorts, used to drive propellors, have been developed since that time for propulsion of aircraft both large and small. Motivation for development has been provided by the military, commerical interests, and by sports enthusiasts.
During the Second World War gas turbine engines were developed for aircraft propulsion. Impetus was primarily military, for increased speed, rate of climb, and payload capability. Gas turbine engines, popularly called jets along with other types of reaction thrust engines, have since become the preferred engines for large aircraft. Today virtually all combat aircraft and large commerical transport aircraft are gas turbine powered.
In the area of civilian aircraft, for large commercial transport, the need is for speed and cargo capacity, to move a large weight of cargo or number of people quickly from one place to another. The turbine engine has proved to be the most cost effective of the engine alternatives, because of its low specific weight, i.e. weight to power ratio, and ability to provide very high power output.
Two disadvantages of turbines have prevented their becoming the preferred power plant for smaller commerical and privately owned aircraft. Such aircraft are variously classed as private, short-haul, commuter aircraft, and others. The first disadvantage is that turbine engines are in general relatively more expensive to design and manufacture than piston engines, because of extremely high rotary speeds and high temperatures compared to piston engines. The other disadvantage in small engines is that gas turbines use fuel at a relatively high rate. They are not fuel efficient. For smaller aircraft, the primary criteria for a power plant is often fuel efficiency. Fuel efficiency is most particularly an important criteria for privately owned and sport aircraft because of weight savings.
For these reasons, piston engines, which are relatively less expensive to manufacture and more fuel efficient than gas turbine and other types of thrust reation engines, are still the preferred engines for small aircraft today. Nearly all small aircraft engines are four-stroke, air-cooled, spark-ignition engines. There are many reasons this is true. Among them are the fact four stroke engines use a full stroke for exhaust and another for intake and are thus more efficiently aspirated than two stroke engines. This fact contributes both to fuel efficiency and to controllability. Precise control and ability to provide excess power above normal cruise conditions for take-off and climbing are important. Another is that spark ignition contributes to controllability by making ignition timing relatively easy to accomplish as opposed to compression-firing. Timing is very important in aircraft engines, particularly under heavy load and power conditions such as at take-off and during climbing. In addition, water cooling systems are generally bulky and add considerably to the gross weight of an aircraft, reducing net load carrying capacity and hence lower fuel efficiency.
In comparison of power plants for fuel efficiency, an often used characteristic is specific fuel consumption, henceforth sfc, which is the weight of fuel used per hour per horsepower produced. Considering power plants in general, a well controlled stationary diesel plant may in best case have an sfc of about 0.32. In aircraft, the larger piston, carbureted, spark ignition engines have the lowest sfc, but under 0.40 is rare. The well known voyager aircraft that flew around the world on a single load of fuel had an sfc of 0.36. In the present invention, prototype testing and computer simulation indicate an sfc of 0.25.
It has been recognized in the art, that if two-cycle engines could be used, that they would offer an advantage in weight. It has also been proposed to use the piston backside for air pumping and for power. Many two-cycle engines use crankcase compression in combination with valves and baffling to improve scavenging. Engines have been built with combustion occurring in both sides of the same cylinder. In that case complex systems to water cool the cylinders are required.
Beyond the goal of low specific fuel consumption, there are other problems with conventional piston-type aircraft power plants. One of these, not limited to aircraft engines, is that the overall thermodynamic cycle efficiency of a piston engine is related to the temperatures at the beginning and end of the compression stroke. For the idealized OTTO cycle, which describes the process of spark ignition piston engines, E=1-T1/T2, where E is efficiency and T1 and T2 are the air-fuel mixture temperatures at the beginning of the compression stroke and at the end of the compression stroke, respectively. Efficiency is enchanced either by lowering T1 or increasing T2. T2 is limited by fuel preignition in spark ignition engines and other engines in which fuel is present during the compression stroke. There are material limitations as well.
Another problem is related to the exhausting of piston engines of all types. After the power stroke in a conventional piston engine, the exhaust gases are still at a high pressure relative to the exhaust manifold pressure. When exhaust valves open there is a sudden sonic expansion into the exhaust manifold, with an irreversible energy loss. This energy is originally supplied by the combustion process, and is lost, not converted to work. The sonic expansion is also the source of objectionable engine noise in piston engines. Mufflers are often included in designs to reduce noise, but add weight and reduce performance.
Exhaust turbines are used to recapture some of the heat-provided energy in exhaust streams, but these do not recapture the energy lost by the expansion from the combustion chamber into the exhaust manifold.
What is needed is a two-cycle, compression-fired piston engine with compression control to provide for ignition timing, avoiding complex spark-ignition, expanding the combustion products on the power stroke to substantially the exhaust manifold pressure, and avoiding sonic exhaust expansion. Also, the exhaust should preferrably be combined with external devices to further improve efficiency and to extract the combustion energy, converting it to thrust for propulsion, thereby providing an improved, i.e. lowered, sfc.